The present invention relates to the field of transmission by optical fiber and more specifically to a line fiber for long-distance applications.
An optical fiber conventionally consists of a central core having the function transmitting and possibly amplifying an optical signal, and an outer optical cladding, having the purpose of confining the optical signal within the central core. For this purpose, the refractive indices of the central core nc and the outer optical cladding ng are such that nc>ng.
For optical fibers, the index profile is generally qualified as a function of the shape of a graph showing the relation between fiber radius and refractive index. Conventionally, the x-axis gives the distance r from the centre of the fiber while the y-axis shows the difference between the refractive index and the refractive index of the fiber cladding. Thus speaking of a “step”, “trapezium” or “triangle” index profile for graphs which show respective step, trapezium or triangular shapes. Such curves are generally representative of the theoretical or set profile for the fiber, whereas the fiber production constraints may lead to a slightly different profile.
There are two main types of optical fibers: multimode and single mode optical fibers. In a multimode fiber, for a given wavelength, several optical modes propagate simultaneously along the fiber. In a single mode optical fiber, the signal propagates in a fundamental mode LP01 guided within the fiber core while higher order modes such as mode LP11 are heavily attenuated. As line fibers for optical fiber transmission systems, step index single mode fibers (SMF) are conventionally employed.
The fiber is typically produced from silica, either natural or synthetic. To obtain the relative index difference between the core and cladding, the core index can be raised by doping it with a suitable substance, such as germanium, the cladding remaining in silica. One alternative consists in preserving a core in silica and decreasing the cladding index by doping it with a suitable substance such as for example fluorine. The result is a so-called Pure Silica Core Fiber (PSCF)—i.e. a fiber having a core that is of undoped silica.
Which of the two processes is chosen can depend on the envisaged use of the fiber. For instance, in a radioactive environment, germanium will combine with hydrogen present in the atmosphere and lead to significant fiber defects thereby increasing transmission losses. A pure silica core fiber with a doped cladding is consequently preferred for use in a radioactive environment. Which process is chosen for obtaining a refractive index profile also depends on the performance required of the fiber. In the particular case of long-distance transmission applications necessitating very low levels of attenuation, pure silica core optical fibers are preferred having an attenuation less than 0.180 dB/km@1550 nm. In effect, the dopant present in the doped core of a fiber contributes to the Rayleigh diffraction (˜0.02 dB/km@1550 nm).
Nevertheless, it is difficult to obtain fibers having a doped cladding. The cost of producing a fiber with a doped cladding is higher than that for a fiber having a cladding in pure silica. Consequently, efforts are made to minimize the radius of the doped cladding in a pure silica core fiber. One solution consists in depositing a doped cladding, called the inner cladding, around a pure silica core, and using standard silica for a part of the cladding called the outer optical cladding. Nevertheless, this method is limited as the outer optical cladding, which has a refractive index close to that of the core, needs to be placed sufficiently far away from the latter. In effect, the proximity of the silica cladding brings about an increase in fundamental propagation mode LP01 leakage losses.
It is known to improve the optical parameters of a fiber using a depressed trench included in the inner cladding between the central core and the outer optical cladding. However, the fact of adding a depressed trench introduces the propagation of additional modes, LP11 principally, which leads to an increase in the cable cut-off wavelength of the fiber. A high cable cut-off wavelength (beyond 1550 nm) limits the single mode character of the fiber with respect to wavelengths of the optical signal.
U.S. Pat. No. 4,852,968 discloses in FIG. 4 a fiber having a core, a first inner cladding, a depressed trench, a second inner cladding and an outer cladding. This document sets out to improve certain optical parameters of the fiber through the presence of a depressed trench and notably, parameters for dispersion, confinement and bending losses of the fundamental mode. But this document does not give details of the characteristics of the depressed inner cladding and of the depressed trench making it possible to obtain low leakage losses for the fundamental mode and a low cut-off wavelength. Additionally, the relative core radii concerning FIG. 4 are relatively small (between 2.5 μm and 3.5 μm). The characteristics of the fiber do consequently not allow it to provide minimized doped cladding radius while preserving low fundamental mode leakage losses and a low cable cut-off wavelength (less than 1550 nm).
U.S. Pat. No. 5,044,724 discloses in FIG. 4 the possibility of employing the profiles discussed in the above document (U.S. Pat. No. 4,852,968) for obtaining structures that are even more depressed with cores having indices close to that of silica. This document does not give details of the characteristics of the depressed inner cladding and of the depressed trench making it possible to obtain a minimized doped cladding radius while preserving low fundamental mode leakage losses and low cut-off wavelengths.
US-A-2005/0089289 discloses, in FIGS. 6 and 8, a silica core fiber having an inner cladding, a depressed trench constituting the periphery of the inner depressed cladding, and an outer cladding. If the cable cut-off wavelength is specified to be below 1330 nm, this document does not give details of the characteristics of the depressed inner cladding and of the depressed trench making it possible to obtain low fundamental mode leakage losses. Additionally, the ratio between the radii of the core and the depressed inner cladding is very small (between 1.25 and 3.34) for this configurations in which the trench is at the periphery of the inner cladding. Similarly, the index difference of the trench compared to that of the outer cladding is fairly high (greater than −7.3×10−3). The characteristics of this fiber do not consequently allow it to provide a minimized doped cladding radius while preserving low fundamental mode leakage losses.
US-A-2008/0279517 discloses a fiber having a core, an inner cladding, a depressed trench constituting the periphery of the depressed inner cladding, and an outer cladding. If the cable cut-off wavelength is less than 1530 nm, no silica core type depressed structure is mentioned and nor, as a consequence, is the impact of a depressed inner cladding and a depressed trench on fundamental mode leakage losses. Further, the radius of the trench located at the periphery of the inner cladding is fairly small (<28 μm), which does not make it possible to obtain low fundamental mode leakage losses.
None of the prior art documents identified seems to disclose a refractive index profile for an optical fiber having a reduced doped cladding radius, exhibiting low fundamental mode leakage losses and a low cut-off wavelength making it possible to maintain single mode character around 1550 nm.
There is consequently a need for an optical fiber having a doped cladding the radius of which is minimized without at the same time increasing fundamental mode LP01 leakage losses, and while preserving a low cut-off wavelength.